Manipulation of long-lived triplet excitons in organic molecules is key to applications including nextgeneration optoelectronics, background-free bioimaging, information encryption, and photodynamic therapy. However, for organic room-temperature phosphorescence (RTP), which stems from triplet excitons, it is still difficult to simultaneously achieve efficiency and lifetime enhancement on account of weak spin-orbit coupling and rapid nonradiative transitions, especially in the red and near-infrared region. Herein, we report that a series of fluorescent naphthalimides-which did not originally show observable phosphorescence in solution, as aggregates, in polymer films, or in any other tested host material, including heavy-atom matrices at cryogenic temperatures-can now efficiently produce ultralong RTP (f = 0.17, t = 243 ms) in phthalimide hosts. Notably, red RTP (l RTP = 628 nm) is realized at a molar ratio of less than 10 parts per billion, demonstrating an unprecedentedly low guest-to-host ratio where efficient RTP can take place in molecular solids.

Organic luminogens with persistent room‐temperature phosphorescence (RTP) have found a wide range of applications. However, many RTP luminogens are prone to severe quenching in the crystalline state. Herein, we report a strategy to construct a donor‐sp3‐acceptor type luminogen that exhibits aggregation‐induced emission (AIE) while the donor‐sp2‐acceptor counterpart structure exhibits a non‐emissive solid state. Unexpectedly, it was discovered that a trace amount (0.01 %) of the structurally similar derivative, produced by a side reaction with the DMF solvent, could induce strong RTP with an absolute RTP yield up to 25.4 % and a lifetime of 48 ms, although the substance does not show RTP by itself. Single‐crystal XRD‐based calculations suggest that n–σ* orbital interactions as a result of structural similarity may be responsible for the strong RTP in the bicomponent system. This study provides a new insight into the design of multi‐component, solid‐state RTP materials from organic molecular systems.

N‐Substituted naphthalimides (NNIs) have been shown to exhibit highly efficient and persistent room‐temperature phosphorescence from an NNI‐localized triplet excited state, when the N‐substitution is a sufficiently strong donor and mediates an intramolecular charge‐transfer (ICT) state upon photo‐excitation. This work shows that, when the electron‐donating ability of the N‐substitution is further increased in the presence of a carbanion or phenoxide, spontaneous electron transfer (ET) occurs and results in radical anions, verified with electron‐paramagnetic resonance (EPR) spectroscopy. However, the EPR‐active anion is surprisingly persistent and impervious to nucleophilic and radical reactions under anionic conditions. The stability is thought to originate from an intramolecular spin pairing between the N‐donor and the NI acceptor post ET, which is demonstrated in supramolecular chemistry.